École Polytechnique Lab Probes Fermion Parity with Josephson Effect

Researchers at École Polytechnique, CNRS have demonstrated a new method for detecting fermion parity, a fundamental property of superconductivity, by utilizing a carbon nanotube-based Andreev molecule. The work employs two coupled quantum-dot Josephson junctions to observe the formation of delocalized Andreev molecular states extending across both junctions, a key step in controlling these complex systems. Changes in the molecular ground-state parity manifest as characteristic phase shifts in the nonlocal response, identifying global fermion parity as an experimentally accessible degree of freedom. This approach could prove relevant for devices designed to encode quantum information in delocalized fermionic degrees of freedom, offering a new pathway for manipulating and reading out quantum states in hybrid superconducting circuits.

The ability to detect the fermion parity of a superconducting state locally represents an advance in manipulating quantum information. Their work, utilizing a uniquely constructed Andreev molecule, reveals how this fundamental property, determining the microscopic nature of a superconducting ground state, can be accessed through a nonlocal Josephson effect. This isn’t simply about confirming parity exists; it’s about establishing a pathway to read it out using a specific, measurable signal. The team fabricated their Andreev molecule from a carbon nanotube hosting two coupled quantum-dot Josephson junctions. The researchers observe a pronounced nonlocal Josephson effect whose magnitude is maximized when the molecular wavefunction becomes delocalized across both junctions. This means the parity isn’t just detected, but its alteration is directly linked to a measurable change in the system’s behavior.

Researchers are now probing fundamental quantum properties using increasingly specific material architectures. A recent study details the creation of Andreev molecules, delocalized superconducting states, using two coupled quantum-dot Josephson junctions fabricated within a single carbon nanotube. This delocalization is key, as it allows for the observation of a pronounced nonlocal Josephson effect, a phenomenon where the supercurrent in one junction is influenced by the phase difference applied to the other. Measurements reveal that changes in the molecule’s ground-state parity are detected, but directly linked to characteristic phase shifts in this nonlocal response. This connection is significant because it allows researchers to infer global fermion parity, a fundamental quantum number, by observing a local measurement. The researchers found this behavior echoes the transition of a single Josephson quantum dot, but manifests in the phase of the nonlocal Josephson signal.

Their work centers on Andreev molecules, created by coupling two Josephson junctions within a single nanotube, and how these structures reveal information about delocalized superconducting states. This approach diverges from traditional methods by utilizing a highly controllable, quasi-one-dimensional platform for investigating complex quantum phenomena. This isn’t simply about creating a superconducting circuit; it’s about engineering a configuration where quantum states aren’t confined to individual components. The team observed a pronounced nonlocal Josephson effect and showed that these shifts in the nonlocal response echo the transition of a single Josephson quantum dot, but appear here in the phase of a nonlocal Josephson signal.

The ability to manipulate and detect fermion parity within quantum systems is moving beyond theoretical curiosity and toward potential applications in quantum information technologies. This isn’t simply a detection of parity, but a demonstration of its active alteration linked to a specific signal change. The experiment leverages a carbon nanotube to create a minimal system where Andreev states hybridize, forming a molecular state sensitive to external control. Crucially, the observed phase shifts provide a direct link between global fermion parity and local measurements.

The expectation that complex superconducting networks would simply amplify the local properties of individual Josephson junctions has proven incomplete; instead, these systems exhibit emergent behaviors stemming from the hybridization of Andreev states. This work, centered around a novel Andreev molecule, moves beyond simply identifying parity to actively probing its influence on circuit behavior. The resulting network isn’t a simple sum of its parts, but a cohesive entity where electron behavior is dictated by the collective state.

The ability to detect and manipulate fermion parity, a fundamental quantum property governing superconducting states, has taken a step forward with the creation of Andreev molecules within carbon nanotubes. Researchers have demonstrated a novel platform for probing this characteristic, utilizing two coupled quantum-dot Josephson junctions fabricated on a single nanotube. The team’s approach diverges from previous implementations relying on extended weak links with multiple conduction channels, which can obscure the connection between transport signatures and the underlying quantum state. Instead, the use of a ballistic, quasi-one-dimensional carbon nanotube ensures a minimal and controllable system. This advancement paves the way for devices that could encode and process quantum information using delocalized fermionic degrees of freedom, potentially revolutionizing quantum computing architectures.

Researchers have demonstrated that changes in the parity of these delocalized superconducting states manifest as measurable phase shifts within the nonlocal Josephson response, offering a new pathway for probing quantum systems. This nonlocal effect is central to the parity detection scheme; however, instead of a current reversal, the parity change appears as a phase shift in the nonlocal Josephson signal.

This is a significant finding, as it allows for the remote detection of a local quantum property. The team’s experimental setup involved a carbon nanotube connecting three superconducting electrodes, allowing for precise electrostatic control. As the paper explains, “the most elementary system that realizes such a network is the Andreev molecule formed in a pair of coupled Josephson junctions.” This minimal design, combined with the ballistic nature of the nanotube, enabled the researchers to unambiguously identify the formation of a delocalized fermionic state and demonstrate that global fermionic parity can be inferred locally through transport measurements.